How are ports used and what exactly are they for?
Queued Callback Ports are necessary to enable a caller to enter the Queued Callback module and schedule a callback. One port is consumed by each caller while their call is queuing. Ports are distributed to callers on a first come first serve basis. Once a caller leaves the queue the port is released. Pending callbacks and the corresponding outbound calls do not consume a port.
How many Queued Callback Ports do I need?
In order to ensure that all callers have the callback option you need to have one port for every simultaneous call in queue. Having a port for each line/channel that feeds into your skill/group will ensure that all callers have the callback option since you can't have more callers in queue than you have phone lines. For example, if you have a single T1-PRI, the max amount of callers you could handle / ports you should consider purchasing is 23. If you aren't sure how many lines actually feed into your skill/group another suitable way to determine the ports you need is to run the Queued Call Volume report and see how high the Max Calls in Queue value reaches.
What happens if I don't have enough ports?
Callers without a port will not hear the Queued Callback invitation and will not be able to enter the Queued Callback module.